The conventional method of rolling hot metal strip involves heating an ingot or slab to approximately 2300.degree. F (for steel) and reducing it in thickness by rolling it through a series of rolling mill stands. Normally the rolling sequence takes place in two stages referred to as a roughing mill and a finishing mill.
In the roughing mill stage the slab or ingot normally is rolled through one or more rolling mill stands in a series of passes until it is reduced in thickness to a transfer bar approximately 1 inch thick. The roughing mill stage also may include one or more vertical edging mills.
Following the roughing operation, the transfer bar normally is transferred on table rolls to a continuous finishing mill train where it is further reduced to the desired gauge.
There are a number of problems inherent in this normal method of rolling hot metal strip. Some of these problems arise from the long length of time that it takes the transfer bar to feed at a relatively slow speed into the finishing mill train. In this connection, the transfer bar is fed into the finishing mill train at a speed that is slower than the speed at which the transfer bar emerges from the roughing mill. Thus, the latter speed may be 600 ft./min. and the former speed 150 ft./min.. The speed of the strip emerging from the finishing mill train is much greater, of course, and may be 2800 ft./min., for example. Another problem is that to provide sufficient future capacity it is necessary to build a mill having greater capacity than that which will be utilized initially.
Because of the high heat transfer rate of the relatively thin transfer bar, the fact that heat is imparted to the transfer bar in the finishing mill, and the fact that the tail end of the transfer bar cools off as the head end thereof passes through the finishing mill train, a considerable temperature drop results between the head and tail ends of the transfer bar during the finishing mill operation. In addition, a considerable amount of secondary scale is formed on the very large exposed surface area of the transfer bar while it is waiting on the delay table ahead of the finishing mill stage. It will be understood that the aforesaid temperature differential creates a problem in that temperature is an important factor in the rolling operation, and changes in temperature must be compensated for if constant strip thickness is to be achieved. Moreover, in order to obtain constant metallurgical properties, strip temperature out of the last finishing mill stand must be kept substantially constant.
In order to overcome the temperature differential problem, modern mills are powered to roll the transfer bar at its minimum tail end temperature, are designed for high speed operation to minimize the time that the transfer bar sits on the delay table and are equipped to provide zoom rolling in order to maintain an acceptable constant strip temperature out of the last finishing mill stand. Zoom rolling involves accelerating the finishing mill after the head end of strip has reached the coilers to compensate for the temperature differential by increasing the amount of heat put into the transfer bar during the finishing mill operation. Zoom rolling also decreases the time that the transfer bar sits on the transfer table. Where zoom rolling in used, zoom cooling also is required.
In order to remove secondary scale formed on the transfer bar while it is waiting on the delay table, a high pressure water descaling unit is employed, this unit being located just ahead of the finishing mill train. Of course, such treatment drastically reduces the temperature of the transfer bar, and additional mill rolling horsepower is required to compensate for this reduction in temperature.
It is known to provide a heat reflector shield over the delay table to reduce the heat radiation loss from the top side of the transfer bar. However, this system only partially conserves the heat of the transfer bar, does not eliminate head to tail rundown or equalize transfer bar temperature and does not prevent formation of secondary scale.
It also is known to roll a tapered transfer bar with its head end thinner than its tail end. The theory of this system is, of course, that the thicker tail end of the transfer bar will lose heat more slowly than the front end thereof and, consequently, reach the first finishing stand at a similar temperature to that of the head end when it was at the entry to the first finishing stand. This technique introduces additional operating variables, e.g., taper rolling in the roughing stands and variable drafting through the finishing stands. It also doesn't prevent formation of secondary scale.
The installation at the delay table of an induction heating furnace to control the temperature of the transfer bar has been suggested. However, this technique could interfere seriously with the removal of cobbles.
The use of a Steckel mill to avoid the aforesaid head to tail temperature differential and its associated problems also is known. A Steckel mill is designed primarily for the purpose of rolling light gauge strip on a single stand reversing hot mill. Normally there is provided a reversing roughing stand that reduces a slab to about 1 inch before presenting it to a single stand, reversing, four high roll stand with a hot coiling furnace located on either side thereof. The transfer bar is passed back and forth through the latter stand until the desired thickness is obtained, the strip being successively reheated in the coiling furnaces on the final passes. This method suffers from the following drawbacks:
a. poor strip surface quality resulting from the formation of scale during the rolling and reheating cycles, this scale being rolled into the strip,
b. fast deterioration of mill work rolls caused by rolled in scale and all work being done on one set of mill rolls, and
c. variation in gauge due to the ends of the strip being colder than the middle of the strip because of the relatively cool temperature of the mandrels and the length of time that the ends of the strip are out of the hot coiling furnaces during the reversing cycle.
In copending British Complete Application Ser. No. 52995/1971 filed Nov. 15, 1972 and based on a British Provisional filed Nov. 15, 1971, there is described, inter alia, a mandrelless downcoiler intended to be inserted in a rolling mill subsequent to the rolling of the transfer bar and prior to the entry of the transfer bar into the finishing mill train. Essentially, the mandrelless coiler construction disclosed in the aforementioned British Application Ser. No. 52995/1971 includes nip rollers for positively feeding the transfer bar toward a set of bending rolls which are positively driven at the same speed as the pinch rolls and which give to the transfer bar a continuous bend or curve. The leading curl on the transfer bar which results from the bending rolls settles against suitable support rollers and constitutes the inside convolution about which the remainder of the transfer bar automatically coils itself. When the tail end has passed the location of the bending rolls, the support rollers on which the coil rests are braked to a stop, while at the same time the tail end of the transfer bar "flops down" against additional aligned rollers, pointing in the same direction as the initial direction of movement of the transfer bar prior to entry into the mandrelless coiler. The support rollers under the coil are then driven in the reverse direction, and the new leading end (previously the tail end) of the coil is fed along a continuing bed of rollers to enter the finishing mill train.
The foregoing mandrelless downcoiler construction offers a number of advantages, among which are: reducing the length of the mill, buildings, foundations, etc. that would otherwise be required; increasing the capacity of an existing mill to roll larger size coils than it was designed to roll; conservation of the heat of the hot metal workpiece; substantial equalization of the temperature of the hot metal workpiece; reduction of the formation of secondary scale on the hot metal workpiece; and reduction in the cost of mill drives, electric motors, power supplies, controls and other electrical equipment.
While there is no question that the foregoing mandrelless downcoiler construction described and clearly set forth in British Complete Application No. 52995/1971 presents an improvement over the prior art techniques and apparatus described earlier in this specification, there is nonetheless some room for further improvement, particularly relating to the most efficient use of the mandrelless downcoiler principle.